and k are identical or different integers from 1 to 6, w is a number assuming the integral values of 0 to 3, R1 is a hydrocarbylene bivalent radical from C1 to C8 which can be substituted by one or more functional groups, and the symbol M, which can be different from one Z group to the other, represents an alkaline metal atom, a radical +NH3 -NH2 or a radical of the formula ##STR20## wherein the s